abi_vectorcall

The tracking issue for this feature is: #124485

Adds support for the Windows "vectorcall" ABI, the equivalent of __vectorcall in MSVC.

extern "vectorcall" {
    fn add_f64s(x: f64, y: f64) -> f64;
}

fn main() {
    println!("{}", add_f64s(2.0, 4.0));
}